NSFetchedResultsController does not always call didChangeObject: atIndexPath: forChangeType: newIndexPath: for NSFetchedResultsChangeMove
I am using NSFetchedResultsController
c sortDescriptors
in a query to populate a table with many results. I notice that when a change occurs that moves a row from the bottom of the table to the top, didChangeObject:atIndexPath:forChangeType:newIndexPath:
it is not called at all.
The weird thing is, I can get around this by iterating over all the selected objects and accessing any attribute on them right after the call performFetch
.
Any advice on what might be the problem, or is this just an obscure Apple bug?
Here is my code:
NSManagedObjectContext *context = [self managedObjectContext];
NSFetchRequest *request = [[NSFetchRequest alloc] init];
request.entity = [NSEntityDescription entityForName:@"MyObject" inManagedObjectContext:context];
request.sortDescriptors = @[NSSortDescriptor sortDescriptorWithKey:@"order" ascending:NO]];
request.fetchBatchSize = 20;
NSFetchedResultsController *fetched = [[NSFetchedResultsController alloc]
initWithFetchRequest:request
managedObjectContext:context
sectionNameKeyPath:nil
cacheName:nil];
fetched.delegate = self;
NSError *error = nil;
if (![fetched performFetch:&error]) {
NSLog(@"Unresolved error fetching objects: %@", error);
}
// Should not be necessary, but objects near the bottom won't move to the top without it.
for (MyObject *o in fetched.fetchedObjects) {
o.someAttribute;
}
Updated September 12, 2014:
I am storing all data in the context of a background managed object and seems to be related to the problems I see. Here is my code to merge changes into the main context of an object:
+(void)initSaveListener {
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(mergeChanges:)
name:NSManagedObjectContextDidSaveNotification
object:[self privateContext]];
}
+(void)mergeChanges:(NSNotification*)notification {
NSManagedObjectContext *context = [self mainContext];
[context performBlock:^{
[context mergeChangesFromContextDidSaveNotification:notification];
NSError *error = nil;
if (![context save:&error]) {
NSLog(@"error merging changes %@, %@", error, [error userInfo]);
}
}];
}
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It turns out that this problem was caused by the changes to being managedObjectContext
propagated from a different context using NSManagedObjectContextDidSaveNotification
. This blog post explains in detail why this is causing the problem for NSFetchedResultsController
and how to fix it:
http://www.mlsite.net/blog/?p=518
Here's a specific fix in the context of my code above:
+(void)mergeChanges:(NSNotification*)notification {
NSManagedObjectContext *context = [self mainContext];
// Fault all objects that have changed so that NSFetchedResultsController will see the changes.
NSArray *objects = [[notification userInfo] objectForKey:NSUpdatedObjectsKey];
for (NSManagedObject *object in objects) {
[[context objectWithID:[object objectID]] willAccessValueForKey:nil];
}
[context performBlock:^{
[context mergeChangesFromContextDidSaveNotification:notification];
NSError *error = nil;
if (![context save:&error]) {
NSLog(@"error merging changes %@, %@", error, [error userInfo]);
}
}];
}
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You are creating a completely new results controller. So there was no change (e.g. insert, delete, update), so no delegate is called. There are two ways to handle this.
You can first use an existing FRC and just change the predicate of its fetch query (the fetch query itself readonly
). Then you just call performFetch
. This may be sufficient depending on your needs.
Second, if you need to erase the FRC and create a new one, you need to call reloadData
on the table view. I usually do this by changing the FRC creation logic through some ivars (after Apple template, FRC is lazy created) and just set FRC to nil
and call [self.tableView reloadData];
.
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